The present invention relates to rubber compositions intended for use in the manufacture of tires.
More particularly, the invention provides colored rubber compositions intended to form at least part of the outer surface of tires. The term xe2x80x9ccoloredxe2x80x9d is understood here to exclude the color black, but to include all colors, including white.
The compositions of the invention are intended to cover the surfaces of tires at least partially, whatever the nature and the composition of the rubber mixes on which they are deposited. Since the compositions form a surface covering, they are more particularly intended to cover what are called xe2x80x9cnon-wearingxe2x80x9d surfaces of tires, that is to say those which are not in contact with the road under normal conditions of use. Thus, such colored compositions according to the invention, which are in particular for decorative and marking purposes, may constitute, at least in part, the outer surface of a tire sidewall or the bottoms of tread patterns of the tread.
It is known that, vulcanized rubber compositions of essentially unsaturated natural and synthetic diene rubbers, because of the presence of double bonds in their molecular chains, are likely to deteriorate more or less rapidly after prolonged exposure to the atmosphere, if they are not protected, as a result of known oxidation mechanisms. These complex mechanisms have been described, for example, in the following documents: ref. [1]: xe2x80x9cAntidegradants for tire applicationsxe2x80x9d in xe2x80x9cTire compoundingxe2x80x9d, Education Symposium No. 37 (ACS), Cleveland, Communication I, October 1995; and [2]: xe2x80x9cAntioxidantsxe2x80x9d in Encycl. Polym. Sci. and Eng., 2d Edition, Vol. 2, 73-91.
It has been gradually possible to inhibit these oxidation phenomena as a result of the development and sale of various antioxidants, the most effective of which are, known derivatives of quinoline (xe2x80x9cTMQxe2x80x9d), or derivatives of p-phenylenediamine (xe2x80x9cPPDxe2x80x9d or xe2x80x9cPPDAxe2x80x9d), which are even more active than the former, such as, for example, N-1,3-dimethylbutyl-Nxe2x80x2-phenyl-p-phenylenediamine (6-PPD). These TMQ and PPD-type antidegradants, singly or in combination with each other, have found widespread systematically use (see, e.g., refs. [1] and [2] above) in conventional tire rubber compositions, filled at least in part with carbon black, which imparts thereto their characteristic black color.
However, the above-mentioned antioxidants, particularly the TMQ or PPD derivatives, are not light-fast. Under the action of UV radiation, they produce colored chemical types which cause an adverse color change of the rubber compositions, staining them dark brown. It is clear that this characteristic rules out the use of such agents within white, clear or colored compositions. Moreover, such characteristics of these antioxidants also cause problems when used in compositions that come into contact with white, clear or colored compositions. If nothing is done to prevent it, the antioxidants will naturally migrate into and onto the surface of the white, clear or colored compositions and stain them.
This is why it is extremely complicated to develop colored compositions intended to cover part of the outer surface of tires and to resist the staining due to the antioxidants present in the other compositions used in tires.
The majority of the solutions that have been considered are based on the use of constituents, that have impereability properties, which may be introduced into rubber compositions. In the tire field, it is known to make use of the impermeability properties of butyl rubber. However, using butyl rubber also creates a number of problems.
In order to make it possible to produce white or colored sidewalls, it has been envisaged to modify the structure of the tires by providing xe2x80x9cscreenxe2x80x9d compositions containing a large amount of butyl rubber in order to prevent the migration of the staining antioxidants. Such screen compositions are intended to separate the white compositions from the other compositions making up said tires. It will readily be understood that such embodiments are liable to adversely affect the properties and performance of the tires concerned, to the extent that significant modifications are made to their structure. Furthermore, it is known that large proportions of butyl rubber in compositions create problems of adhesion of the latter to the other compositions present in the tire.
Other solutions have consisted of directly introducing butyl rubber into the colored compositions. Thus publication CA-2 228 692 describes a composition for white sidewalls which uses an elastomeric base containing a majority of butyl rubbers in order to constitute a barrier to the staining antioxidants. These solutions make it possible to avoid certain modifications of the structure of the tire, but do not rectify the problems linked to adhesion. This is why, since the interface obtained between white sidewalls and the other mixes present in the tire is fragile, such compositions for white sidewalls can only be deposited on very limited non-stressed parts of the sidewall, in order to avoid detachment at interfaces of the compositions.
Additional solutions involve producing compositions intended to be used within the mass and not just at the surface, which abandon the use of the antioxidants based on PPD or TMQ derivatives within these compositions. However, because such antioxidants are very effective, replacing them requires providing complex protective systems, such as those described in the publication WO 99/02590. It should furthermore be noted that it is not generally possible, in all the compositions present in the tire, to replace the antioxidants based on PPD or TMQ derivatives, which are very high-performance. Thus, because of their migration, it is necessary to provide a solution, such as that using butyl rubber, the drawbacks of which have already been mentioned.
The invention is directed to colored compositions capable of forming part of the outer surface of tires, which overcome the disadvantages of prior compositions.
The Inventor has surprisingly discovered that sulphur-vulcanizable, colored rubber compositions having a very high content of non-stoichiometric iron oxide, in an amount of more than 50 phr (parts per 100 parts by weight of elastomer), make it possible to produce colored surface compositions for tires which are capable of being deposited directly on compositions containing staining antioxidants, without requiring the use of barrier agents typical of antioxidants and preserve their surface color.
Because of the known photocatalytic activity of non-stoichiometric iron oxides which cause degradation of organic or biorganic molecules, the existence of such properties a priori would prevent the person skilled in the art from using large quantities of these oxides for several reasons: (1) because the properties of degrading double bonds are contrary to desired protective characteristics intended in the tire to avoid this degradation by oxidation, and (2) it is quite surprising that the presence of non-stoichiometric iron oxides alone makes it possible to render the surface of the compositions self-cleaning with such a small amount.
The invention is therefore a sulphur-vulcanizable colored rubber composition comprising at least one elastomeric base, at least 30 parts of which per 100 parts by weight of elastomer (phr) comprises an essentially unsaturated diene elastomer, the composition having a content of at least 50 phr of non-stoichiometric iron oxide. In particular, the non-stoichiometric iron oxide may be non-stoichiometric iron monoxide or non-stoichiometric iron (III) oxide, Fe2O3.
xe2x80x9cDienexe2x80x9d elastomer or rubber is understood to mean, in known manner, an elastomer resulting at least in part (i.e. a homopolymer or a copolymer) from diene monomers (monomers bearing two double carbon-carbon bonds, whether conjugated or not).
Generally, xe2x80x9cessentially unsaturatedxe2x80x9d diene elastomer is understood to mean a diene elastomer resulting at least in part from conjugated diene monomers having a content of members or units of diene origin (conjugated dienes) which is greater than 15% (mole %).
Thus, for example, diene elastomers such as butyl rubbers or copolymers of dienes and of alpha-olefins of the EPDM type do not fall within the preceding definition, and may in particular be described as xe2x80x9cessentially saturatedxe2x80x9d diene elastomers (low or very low content of units of diene origin which is always less than 15%).
Within the category of xe2x80x9cessentially unsaturatedxe2x80x9d diene elastomers, xe2x80x9chighly unsaturatedxe2x80x9d diene elastomer is understood to mean a diene elastomer having a content of units of diene origin (conjugated dienes) which is greater than 50%.
These definitions being given, the following are understood to be meant by essentially unsaturated diene elastomer capable of being used in the compositions according to the invention:
(a) any homopolymer obtained by polymerization of a conjugated diene monomer having 4 to 12 carbon atoms; or
(b) any copolymer obtained by copolymerization of one or more dienes conjugated together or with one or more vinyl aromatic compounds having 8 to 20 carbon atoms.
Suitable conjugated dienes are, in particular, 1,3-butadiene, 2-methyl-1,3-butadiene, the 2,3-di(C1 to C5 alkyl)-1,3-butadienes such as, for example, 2,3-dimethyl-1,3-butadiene, 2,3-diethyl-1,3-butadiene, 2-methyl-3-ethyl-1,3-butadiene, 2-methyl-3-isopropyl-1,3-butadiene, an aryl-1,3-butadiene, 1,3-pentadiene and 2,4-hexadiene.
Suitable vinyl aromatic compounds are, in particular, styrene, ortho-, meta- and para-methylstyrene, the commercial mixture xe2x80x9cvinyl-toluenexe2x80x9d, para-tertiobutylstyrene, the methoxy-styrenes, the chloro-styrenes, vinyl mesitylene, divinyl benzene and vinyl naphthalene.
The copolymers comprise between 99% and 20% by weight of diene units and between 1% and 80% by weight of vinyl aromatic units. The elastomers may have any microstructure, which is a function of the polymerization conditions used, in particular of the presence or absence of a modifying and/or randomizing agent and the quantities of modifying and/or randomizing agent used. The elastomers may, for example, be block, statistical, sequenced or microsequenced elastomers, and may be prepared in dispersion or in solution.
Polybutadienes are preferred, particularly those having a 1,2 bond content of between 4% and 80% and those having a content of cis-1,4 bonds of more than 90%, polyisoprenes; butadiene-styrene copolymers, and in particular those having a styrene content of between 5% and 50% by weight and, more particularly, between 20% and 40% by weight, a 1,2 bond content of the butadiene part of between 4% and 65%, and a content of trans-1,4 bonds of between 20% and 80%; butadiene-isoprene copolymers, particularly those having an isoprene content of between 5% and 90% by weight and a glass transition temperature (Tg) of xe2x88x9240xc2x0 C. to xe2x88x9280xc2x0 C. and isoprene-styrene copolymers, particularly those having a styrene content of between 5% and 50% by weight and a Tg of between xe2x88x9225xc2x0 C. and xe2x88x9250xc2x0 C.
In the case of butadiene-styrene-isoprene copolymers, there are suitable in particular those having a styrene content of between 5% and 50% by weight and, more particularly, between 10% and 40%, an isoprene content of between 15% and 60% by weight, and more particularly between 20% and 50%, a butadiene content of between 5% and 50% by weight, and more particularly between 20% and 40%, a content of 1,2-units of the butadiene part of between 4% and 85%, a content of trans-1,4 units of the butadiene part of between 6% and 80%, a content of 1,2- plus 3,4-units of the isoprene part of between 5% and 70%, and a content of trans-1,4 units of the isoprene part of between 10% and 50%, and more generally any butadiene-styrene-isoprene copolymer having a Tg of between xe2x88x9220xc2x0 C. and xe2x88x9270xc2x0 C., and more generally any butadiene-styrene-isoprene copolymer having a Tg of between 20xc2x0 C. and xe2x88x9270xc2x0 C.
In particular, the diene elastomer of the composition of the invention is selected from among strongly unsaturated diene elastomers which include polybutadienes (BR), polyisoprenes (IR) or natural rubber (NR), butadiene-styrene copolymers (SBR), butadiene-isoprene copolymers (BIR), isoprene-styrene copolymers (SIR), butadiene-acrylonitrile copolymers (NBR), isoprene-styrene copolymers (SIR) [sic], butadiene-styrene-isoprene copolymers (SBIR), or a mixture of two or more of these compounds.
Such diene elastomers may be used alone or in a blend with other elastomers conventionally used in tires, such as the diene elastomers comprising:
a ternary copolymer obtained by copolymerization of ethylene, and an xcex1-olefin having 3 to 6 carbon atoms with a non-conjugated diene monomer having 6 to 12 carbon atoms, such as, elastomers obtained from ethylene or from propylene with a non-conjugated diene monomer of the afore-mentioned type, particularly 1,4-hexadiene, ethylidene norbomene or dicyclopentadiene;
a copolymer of isobutene and isoprene (butyl rubber or IIR), as well as the halogenated, particularly chlorinated or brominated, versions (BIIR), of this copolymer; or
a copolymer of isobutene and paramethylstyrene, as well as halogenated, particularly chlorinated or brominated, versions (BIMS), of this type of copolymer.
It is clear that the photocatalytic properties of the non-stoichiometric iron oxides can only be used effectively where there is a sufficient presence of elastomers having unsaturated bonds liable to be degraded, e.g., in the order of 30% of the elastomeric base. Furthermore, in order to be able to deposit the colored compositions of the invention on any non-wearing tire surface as described above, it is necessary that the elastomeric base of the non-wearing tire surface primarily comprise such essentially unsaturated diene elastomers.
Preferred non-stoichiometric iron oxides are non-stoichiometric iron monoxides and non-stoichiometric iron (III) oxide (Fe2O3).
Since iron monoxide is yellow and Fe2O3 is red in color, they may be used alone, together or in the presence of other colored pigments or optical brightening agents in order to obtain yellow or other colors, bearing in mind that the elementary rules of color mixing apply.
Thus it is possible to use any type of coloring agent known to the person skilled in the art, including coloring agents that are organic or inorganic, and soluble or insoluble, in the compositions according to the invention. Non-limiting examples include mineral colorants, such as, powdered metals, in particular powdered copper or aluminum, various metal oxides, in particular silicates, aluminates or titanates, iron oxides or hydroxides, mixed oxides of different metallic elements such as Co, Ni, Al or Zn. Organic pigments such as indanthrones, diketo-pyrrolo-pyrroles or diazo condensates, or organometallic pigments such as phthalocyanines are also considered.
The compositions may also contain a reinforcing filler such as carbon black or silica to impart other types of properties to the compositions according to the invention. However, it is important to consider the color of the filler, which may modify or adversely affect the desired coloration of the composition.
With consideration of the above-described aspects of coloration, the compositions according to the invention contain, in addition to the compounds previously described, all or part of the constituents usually used in diene rubber compositions for tires, such as plasticizers, a cross-linking system based either on sulphur or on sulphur and/or peroxide donors, vulcanization accelerators, extender oils, of the aromatic, naphthenic or paraffinic type, and also various anti-fatigue agents.
The rubber compositions are prepared using the diene elastomers according to well known techniques, by thermomechanical working in one or two stages in an internal paddle mixer, followed by mixing on an external mixer.
According to conventional techniques, a conventional internal mixer is used to mix the elastomers, the filler and the other constituents of the composition with the exception of the vulcanization system. The mixture obtained then taken up on an external mixer, generally an open mill, and then the vulcanization system is added thereto. One or more stages may be added in the internal mixer, essentially with the aim of making the mixture undergo complementary thermomechanical treatment.